[RealJames]

I had my experience, no paper qualifications.
My first job at NOVA gave me a 1 day training course, which was pure and utter nonsense and had no bearing on any of the attendees ability to teach.

In other words, experience.
You can't teach private lessons for a year, as a full time job, without learning all the grammar and tricks of the trade required to be a decent teacher.
You'll learn more that way also than any certificate will give.

I hired a teacher with no certs, but 4 years experience with teaching in Japan, and I chose him over someone with a list the length of my arm of certificates and papers but had no experience in Japan, just in Spain.

When I first started teaching English, it amazed me how many grammatical rules there are which we native speakers use every day naturally. Then learning how to explain them was the first hurdle, then each different nationality of people I taught English to had entirely different stumbling blocks and the stuff I'd learned in the previous was largely unnecessary lol.